A fasting-related change in drug exposure can arise through absorption or through removal from the circulation. A crossover study separated these questions by testing oral and intravenous drug probes after different fasting periods.
Twelve healthy adults received a probe cocktail after an overnight fast and after 36 hours of fasting. The researchers estimated systemic clearance and oral bioavailability. Caffeine clearance increased by about 17% and metoprolol clearance by about 13%, while S-warfarin clearance decreased by about 19%. They did not detect a change in oral bioavailability.
The directions differed across the probes. A reconstruction should identify the actual medicine, exposure conditions and evidence relevant to its clearance. This short study in healthy volunteers does not supply a universal adjustment for prolonged malnutrition, illness or every CYP-metabolized drug.